An applicant of the present invention has a patent right of an antenna device, in which a substantially conductive member is provided around a microstrip antenna, in Japan (Japanese Patent No. 3026171.
In the antenna device of Japanese Patent No. 3026171, it is intended to improve gain and to realize a narrower beam width (here, a beam width represents a half-power width), when compared with the case where a substantially cylindrical conductive member is not provided around a microstrip antenna.
More concretely, whereas although the gain of the conventional microstrip antenna is about 7 dBi, in the above-mentioned antenna device, it is intended to increase gain and to realize a narrower beam width such that a substantially cylindrical conductive member is provided around a microstrip antenna in contrast to an conventional microstrip antenna characterized in that the thickness of the antenna is small, that the antenna is light, that the structure of the antenna is simple, and that a circularly polarized wave can be easily obtained. As a result, although dependent on the height and diameter of a substantially cylindrical conductive member, for example, an antenna device having a gain of about 9 dBi or more and a beam width of about 50 degrees can be obtained.
It is an object of the present invention to provide an antenna device having a high gain and/or a narrow beam width such that an antenna device shown in Japanese patent No. 3026171 is improved.